The yeast Candida albicans is found as a commensal organism in the digestive tract of mammals. It is also the most common human fungal pathogen, causing both mucosal and systemic infections, particularly in immunocompromised people. This proposal covers studies of six different aspects of Candida five of which have been implicated in aspects of pathogenesis. (The sixth has not yet been studied.) These areas are (1) the reversible blastospore to filament morophologica transitions, (2) specialization of hyphal tip cells, (3) white-opaque switching, (4) mating, (5) the identification of 'virulence' genes and, (6) the possible role of regulated RNA splicing in virulence.